Re: Attempted summary of suspend-blockers LKML thread

From: Arve Hjønnevåg
Date: Tue Aug 03 2010 - 23:39:29 EST

On Tue, Aug 3, 2010 at 5:51 PM, <david@xxxxxxx> wrote:
> On Tue, 3 Aug 2010, Paul E. McKenney wrote:
>> On Tue, Aug 03, 2010 at 04:19:25PM -0700, david@xxxxxxx wrote:
>>> On Tue, 3 Aug 2010, Arve Hj?nnev?g wrote:
>>>> 2010/8/2  <david@xxxxxxx>:
>>>>> so what is the fundamental difference between deciding to go into
>>>>> low-power
>>>>> idle modes to wake up back up on a given point in the future and
>>>>> deciding
>>>>> that you are going to be idle for so long that you may as well suspend
>>>>> until
>>>>> there is user input?
>>>> Low power idle modes are supposed to be transparent. Suspend stops the
>>>> monotonic clock, ignores ready threads and switches over to a separate
>>>> set of wakeup events/interrupts. We don't suspend until there is user
>>>> input, we suspend until there is a wakeup event (user-input, incoming
>>>> network data/phone-calls, alarms etc..).
>>> s/user input/wakeup event/ and my question still stands.
>>> low power modes are not transparent to the user in all cases (if the
>>> screen backlight dimms/shuts off a user reading something will
>>> notice, if the system switches to a lower clock speed it can impact
>>> user response time, etc) The system is making it's best guess as to
>>> how to best srve the user by sacraficing some capibilities to save
>>> power now so that the power can be available later.
>>> as I see it, suspending until a wakeup event (button press, incoming
>>> call, alarm, etc) is just another datapoint along the same path.
>>> If the system could not wake itself up to respond to user input,
>>> phone call, alarm, etc and needed the power button pressed to wake
>>> up (or shut down to the point where the battery could be removed and
>>> reinstalled a long time later), I would see things moving into a
>>> different category, but as long as the system has the ability to
>>> wake itself up later (and is still consuming power) I see the
>>> suspend as being in the same category as the other low-power modes
>>> (it's just more expensive to go in and out of)
>>> why should the suspend be put into a different category from the
>>> other low-power states?
>> OK, I'll bite...
> thanks, this is not intended to be a trap.
>> From an Android perspective, the differences are as follows:
>> 1.      Deep idle states are entered only if there are no runnable tasks.
>>        In contrast, opportunistic suspend can happen even when there
>>        are tasks that are ready, willing, and able to run.
> Ok, this is a complication to what I'm proposing (and seems a little odd,
> but I can see how it can work), but not neccessarily a major problem. it
> depends on exactly how the decision is made to go into low power states
> and/or suspend. If this is done by an application that is able to look at
> either all activity or ignore one cgroup of processes at different times in
> it's calculations than this would work.
>> 2.      There can be a set of input events that do not bring the system
>>        out of suspend, but which would bring the system out of a deep
>>        idle state.  For example, I believe that it was stated that one
>>        of the Android-based smartphones ignores touchscreen input while
>>        suspended, but pays attention to it while in deep idle states.
> I see this as simply being a matter of what devices are still enabled at the
> different power savings levels. At one level the touchscreen is still
> powered, while at another level it isn't, and at yet another level you have
> to hit the power soft-button. This isn't fundamentally different from
> powering off a USB peripheral that the system decides is idle (and then not
> seeing input from it until something else wakes the system)

The touchscreen on android devices is powered down long before we
suspend, so that is not a good example. There is still a significant
difference between suspend and idle though. In idle all interrupts
work, in suspend only interrupts that the driver has called
enable_irq_wake on will work (on platforms that support it).

>> 3.      The system comes out of a deep idle state when a timer
>>        expires.  In contrast, timers cannot expire while the
>>        system is suspended.  (This one is debatable: some people
>>        argue that timers are subject to jitter, and the suspend
>>        case for timers is the same as that for deep idle states,
>>        but with unbounded timer jitter.  Others disagree.  The
>>        resulting discussions have produced much heat, but little
>>        light.  Such is life.)
> if you have the ability to wake for an alarm, you have the ability to wake
> for a timer (if from no other method than to set the alarm to when the timer
> tick would go off)

If you just program the alarm you will wake up see that the monotonic
clock has not advanced and set the alarm another n seconds into the
future. Or are proposing that suspend should be changed to keep the
monotonic clock running? If you are, why? We can enter the same
hardware states from idle, and modifying suspend to wake up more often
would increase the average power consumption in suspend, not improve
it for idle. In other words, if suspend wakes up as often as idle, why
use suspend?

>> There may well be others.
>> Whether these distinctions are a good thing or a bad thing is one of
>> the topics of this discussion.  But the distinctions themselves are
>> certainly very real, from what I can see.
>> Or am I missing your point?
> these big distinction that I see as significant seem to be in the decision
> of when to go into the different states, and the difference between the
> states  themselves seem to be less significant (and either very close to, or
> within the variation that already exists for power saving modes)
> If I'm right bout this, then it would seem to simplify the concept and
> change it from some really foreign android-only thing into a special case
> variation of existing core concepts.

Suspend is not an android only concept. The android extensions just
allow us to aggressively use suspend without loosing (or delaying)
wakeup events. On the hardware that we shipped we can enter the same
power mode from idle as we do in suspend, but we still use suspend
primarily because it stops the monotonic clock and all the timers that
use it. Changing suspend to behave more like an idle mode, which seems
to be what you are suggesting, would not buy us anything.

> you have many different power saving modes, the daemon (or kernel code) that
> is determining which mode to go into would need different logic (including,
> but not limited to the ability to be able to ignore one or more cgroups of
> processes). different power saving modes have different trade-offs, and some
> of them power down different peripherals (which is always a platform
> specific, if not system specific set of trade-offs)

The hardware specific idle hook can (and does) decide to go into any
power state from idle that does not disrupt any active devices.

> This all depends on the ability for the code that decides to switch power
> modes (including to trigger suspend) to be able to see things in sufficient
> detail to be able to do different things depending on the class of programs.
> I don't know enough about this code to know if this is the case or not, I
> really wish that someone familiar with the power saving code could either
> confirm that this is possible, or state that it's not possible (or at least,
> not without major surgery)

Arve Hjønnevåg
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